We outline the methodology of our numerical studies aimed at increasing the understanding of the relation between dynamics and stress field of the lithosphere with particular reference to oceanic lithosphere. The ridge-push force is modelled as a pressure gradient integrated over all contributing parts of the lithosphere. The slab-pull force is modelled as being dependent on the age of the subducting lithosphere. We parametrize the resistive forces and determine the unknown parameters by requiring the total torque of all forces acting on the plate to vanish. We illustrate the approach by the presentation and discussion of new modelling results for the Pacific plate.